An ultrasonic motor (USM) is a motor with a piezoelectric (e.g., lead zirconium titanate (PZT)) actuator operating in the ultrasonic range above the audible band (about 20 kHz) so that it moves silently to humans. A single piezoelectric strip with two partitions is bonded on the bottom to stator. The two partitions are separated by a quarter wavelength spatially and are excited by signals 90 degrees out of phase. A preload applied between stator and rotor converts high frequency, low amplitude vibrations to unidirectional motion via frictional coupling. To amplify motion, the PZT or other piezoelectric element typically excites a system resonance of the USM.
USM's generally include a driver that supplies two high voltage waveforms in quadrature at a frequency above the audible band. This frequency is typically adjusted within a narrow range just above the series resonance frequency in very fine steps. For example, one particular USM may have a series resonance frequency of 61.6 KHz which needs a frequency resolution (Δf) of 5 Hz or better in its application. This translates into a frequency step of about 0.08%. The USM torque when operating above the series resonance frequency is known to vary more smoothly and linearly than below the resonance frequency. Below the resonance frequency, where it drops off precipitously to control the USM below the resonance frequency, one would need even more frequency resolution. Hence, there is a need to adjust the frequency of the motor just above the series resonance frequency.
This needed frequency resolution (Δf) presents a problem for conventional pulse-width modulation (PWM) generators which develop their PWM output frequency using a divide-by-N clock circuit. For these circuits, Δf is given by:
            Δ      ⁢                          ⁢      f        =                  f        pwm        2                    f        clock                        f      pwm        =          PWM      ⁢                          ⁢      output      ⁢                          ⁢      frequency                  f      clock        =          Clock      ⁢                          ⁢      frequency      In this example, obtaining a 5 Hz resolution for a 61.2 kHz PWM output frequency requires a clock frequency (fclock) of 720 MHz, which is beyond the capability of low-cost microcontroller units (MCU's).
Conventionally, to solve this problem to provide the needed frequency resolution for the USM, for example Δf=5 Hz, USM motor driver boards use a digital to analog converter (DAC) that receives a signal from the motor sensed by a servo and velocity control block, followed by a voltage controlled oscillator (VCO) to develop the desired motor driver frequency. The VCO in turn feeds a ring counter to generate quadraphase PWM outputs with respective phase delays of 0, 90, 180, and 270 degrees. These quadraphase PWM outputs are commonly connected to two push-pull power drivers of the motor driver circuit that is coupled to drive the USM.
This conventional solution to the frequency resolution for USM's problem is expensive, requiring at least 3 external components (DAC, VCO, and a ring counter). The frequency resolution achievable for this known arrangement depends on the VCO sensitivity and the number of bits in the DAC.